Production of discoid grinding products consisting of a flexible underlay, such as paper, is well known. One side of the underlay is provided at least with one adhesive layer, which includes an essentially continuous layer of grinding agent substantially applied thereto.
To enable continuous maintenance of the grinding product's grinding effect, as much of the grinding dust released during grinding as possible must be removed from the space between the grinding product and the grinding surface of the object being ground. As the underlay of such grinding products has not conventionally been air and dust permeable, the grinding products have usually been provided with specific perforated openings, which extend through the grinding product and are connected to air ducts, for example, through which grinding dust can be sucked out while grinding continues. Such products are described in EP 0 781 629, for instance.
These prior art grinding products are produced so that an otherwise finished grinding product is fed through an apparatus where the grinding product is provided with holes by perforation or another piercing process. However, this way of providing the grinding product with through-holes has several disadvantages. Perforation causes cracks and other undesired deformations at the edges of the holes, both in the underlay and in its adhesive layer. Due to crack formation and deformations, the material bridges between the holes must be made relatively wide because narrow material bridges can easily break. Consequently, the holes must be placed at a relatively long distance from one another, as a result of which the grinding product is not cleaned efficiently enough and its grinding effect decreases relatively fast during grinding.
Such perforation also causes other disadvantages. Perforation of the underlay weakens the grinding product to such an extent that a continuous grinding belt cannot be formed of it. The reason for this is that perforation causes increased stretching in the underlay when it is subjected to stress. In addition, an edge perpendicular to the main plane of the grinding product is often formed at the hole edges of the underlay in the perforation process. This edge makes the removal of grinding dust through the holes of the grinding product more difficult. It is further generally known that the perforator wears fast during perforation because of the hard grinding agent.
Most of the above-mentioned disadvantages can naturally be avoided by coating the underlay of the grinding product after it has been perforated, impregnated and coated with a sufficient number of adhesive layers as described in U.S. Pat. No. 2,838,890. The problem associated with this solution is that this grinding product cannot be used in producing a continuous grinding belt, or grinding belts made of perforated grinding products can only be produced from grinding products having a relatively thick underlay. The underlay must be thick so as avoid stretching of the grinding belt. However, the impregnation and coating of the underlay in accordance with the above-mentioned US publication makes the underlay brittle, which means that the grinding product can be bent only a little without breaking it. A grinding belt made of this grinding product would thus break easily in use. If the grinding belt is sufficiently elastic so that is does not break, there is the risk that the grinding belt is stretched too much by belt tension.
Net-like grinding products with a cloth base provided with through-holes for dust removal have also been devised. Such a solution is described in WO 96/13358, for example. The problem associated with this solution is that the cloth requires a special production technique and its use is thus expensive. Also the processing and coating of the cloth require special methods. Furthermore, the finished product has the disadvantage that the cut and perforated edges are weakened by the holes that are on the cutting line since they form notches in the edge of the finished product.
Finally, it may be mentioned that the problem arising in connection with the above-mentioned perforation of grinding products has been relieved by using as the underlay an open cloth or a net where most of the surface is air-permeable. Grinding products produced this way have, however, a poorer grinding capacity than the grinding products described earlier because a continuous and substantially even layer of grinding agent cannot be applied to a cloth or a net. The grinding agent layer will not be in a uniform main plane but will follow the uneven surface of the cloth or net in different planes. As the amount of grinding agent acting on the surface of the work piece is smaller per unit area compared to a case where the whole surface is coated with grinding agent, the grinding effect will decrease. Such a grinding product is described in FI 96584, for example.